Artificial satellites are frequently used to implement telecommunications systems. They make it possible notably to cover geographical zones for which terrestrial networks have not been deployed, or else to interconnect distant terrestrial networks. An artificial satellite comprises a payload, that is to say a set of equipment allowing it to perform the operations for which it has been designed. For both technical and also cost reasons, the weight of this payload must be minimized. Thus, processing operations that it would be functionally appropriate to do at the satellite level are sometimes implemented at the level of the terrestrial stations of the system by reason of these constraints.
During the deployment of the system, the satellites are placed at a previously chosen orbit, the choice of this orbit being made during the design of the system. Once in orbit, it is difficult to intervene physically on a satellite, notably in the case of a fault. This must of course be taken into account by the designers, and certain circuits exhibit a certain redundancy so as to be able to replace defective resources.
A telecommunication satellite customarily transmits and receives on several distinct channels, a channel corresponding to a band of frequencies. The payload of the satellites can therefore be channeled, that is to say a set of equipment, such as for example amplifiers, are dedicated to the sending or receiving of signals on a given channel. During transmission, the transmitted signals for each channel are shaped by successions of gains associated with each channel and are multiplexed using an output multiplexer, designated in the subsequent description by the acronym OMUX with reference to the expression “Output Multiplexer”. An OMUX multiplexer corresponds to an assemblage of radiofrequency filters. The signal resulting from the multiplexing is thereafter directed toward a broadband transmission antenna. The filters of the OMUX are, for example, bandpass filters which are allocated to the various transmission channels and make it possible to prevent the signal transmitted on a channel from interfering with the signals of the adjacent channels. A portion of the signal generated by the equipment associated with a channel may be reflected for example at the level of the OMUX multiplexer, this portion corresponding to the frequencies of the signal not belonging to the passband of the filter. These reflections may be the consequence of human error resulting for example from erroneous frequency programming by an operator of a ground control station. These reflections may also be the consequence of malfunctions or poor programming of the equipment of the payload carrying out the steering of the high-power signals output by the gain succession of the channels toward the filters of the OMUX, this equipment being called switches in the subsequent description. Faults may also give rise to reflections, notably if the onboard oscillator used for the frequency transposition of the signal to be transmitted is defective. Moreover, if the satellite is used as a repeater, a fault on the ground involving a shift in the frequency of the signal received by the satellite will give rise to a shift at the level of the transmission channel and therefore reflections at the level of the filters of the OMUX. Moreover, poor adaptation of the output of the repeater may also induce signal reflections.
The radiofrequency power resulting from these reflections must be dissipated so as to guarantee good operation in transmission. For this purpose, existing solutions position high-power isolators (customarily designated by the acronym HPI) composed of a circulator and of a power load, in the successions of radiofrequency gains.
The reflections mentioned induce a dissipation of power in thermal form, and the temperature of the circuits such as the OMUXs and the power loads may increase considerably until they are irreparably damaged.
In order to avoid this, existing solutions for thermal detection propose measuring the temperature at the level of the filters of the OMUX multiplexer with the aid of thermistors, said thermistors generating measurement signals processed by an embedded processor aboard the satellite. The main drawback of these solutions is their reaction time which is of the order of several tens of seconds before the components are adjusted so that the temperature falls.